Separate and joint clustering characteristics of large Stokes number sprays subjected to moderately turbulent co-flows

TL;DR

This study experimentally investigates the clustering and void characteristics of large Stokes number sprays in turbulent co-flows, revealing linear relationships between cluster size and Stokes number and consistent droplet density ratios.

Contribution

It provides new experimental insights into the separate and joint clustering behaviors of droplets, clusters, and voids in turbulent co-flows with large Stokes numbers.

Findings

Cluster length scale varies linearly with Stokes number.

Droplet density in clusters is about 10 times higher than in voids.

Droplet size distribution broadens with increased turbulence.

Abstract

Separate and joint droplets, clusters, and voids characteristics of sprays injected in a turbulent co-flow are investigated experimentally. Simultaneous Mie scattering and Interferometric Laser Imaging for Droplet Sizing along with separate hotwire anemometry are performed. The turbulent co-flow characteristics are adjusted using zero, one, or two perforated plates. The Taylor length scale-based Reynolds number varies from 10 to 38, and the Stokes number estimated based on the Kolmogorov time scale varies from 3 to 25. The results show that the mean length scale of the clusters normalized by the Kolmogorov length scale varies linearly with the Stokes number. However, the mean void length scale is on the order of the integral length scale. It is shown that the number density of the droplets inside the clusters is about 10 times larger than that in the voids. The ratios of the droplets number densities in the clusters and voids to the total number density are independent of the test conditions and equal 8.3 and 0.7, respectively. The joint probability density function of the droplets diameter and clusters area shows that the droplets with the most probable diameter are found in the majority of the clusters. It is argued that intensifying the turbulence broadens the range of turbulent eddy size in the co-flow which allows for accommodating droplets with a broad range of diameters in the clusters. The results are of significance for engineering applications that aim to modify the clustering characteristics of large Stokes number droplets sprayed into turbulent co-flows.